Several double-stranded (ds) RNA viruses of families Totiviridae and Partitiviridae persistently infect parasitic protozoa, including important human pathogens of genera Cryptosporidium (diarrheal disease), Giardia (diarrheal disease), Leishmania (cutaneous, mucocutaneous, and visceral forms of disease), and Trichomonas (genitourinary disease). Recent studies of Leishmania and Trichomonas have provided evidence that their respective totiviruses, Leishmania RNA virus and Trichomonas vaginalis virus (TVV), contribute to the protozoan- associated diseases by inducing proinflammatory responses by mammalian cells, which influence the degree and nature of inflammation in surrounding tissues. Such results and others have led to a general hypothesis that the widely prevalent dsRNA viruses of parasitic protozoa may commonly affect protozoan interactions with mammalian cells in ways that have important consequences for aggravating the respective human diseases or for otherwise enhancing the survival or transmission of these protozoa during their natural life cycles. A broad objective in this emerging field is to gain a better understanding of the effects of protozoal dsRNA viruses on both protozoan and mammalian cells. The current proposal is focused on the four species of TVVs and their protozoan host T. vaginalis, the most common nonviral sexually transmitted pathogen worldwide, which is also associated with increased transmission of HIV, preterm delivery, and low birth weight. T. vaginalis is increasingly recognized as one of the most neglected pathogens of major relevance to human health, especially women's and newborns' health, in the US and around the world. The studies proposed here will enhance understanding of the basic biology of the TVVs and will develop tools, reagents, and targets for ongoing studies of these viruses as pathogenicity determinants for trichomoniasis and its associated complications. The specific aims of this proposal are as follow: Aim 1, to advance structural studies of TVV virions; Aim 2, to characterize dsRNA satellites that depend on one or more of the TVV species for replication and maintenance; and Aim 3, to dissect signals for RNA packaging, synthesis, and translation in the TVV genome.